It is a common requirement in a mass spectrometer for ions to be transferred through a region maintained at an intermediate pressure i.e. at a pressure wherein collisions between ions and gas molecules are likely to occur as ions transit through an ion guide. Ions may need to be transported, for example, from an ionisation region which is maintained at a relatively high pressure to a mass analyser which is maintained at a relatively low pressure. It is known to use a radio frequency (RF) transportion guide operating at an intermediate pressure of around 10−3 to 10−1 mbar to transportions through a region maintained at an intermediate pressure. It is also well known that the time averaged force on a charged particle or ion due to an AC inhomogeneous electric field is such as to accelerate the charged particle or ion to a region where the electric field is weaker. A minimum in the electric field is commonly referred to as a pseudo-potential well or valley. Known RF ion guides are designed to exploit this phenomenon by creating a pseudo-potential well wherein the minimum of the pseudo-potential well lies along the central axis of the ion guide and wherein ions are confined radially within the ion guide.
It is known to use an RF ion guide to confine ions radially and to subject the ions to Collision Induced Dissociation or fragmentation within the ion guide. Fragmentation of ions is typically carried out at pressures in the range 10−3 to 10−1 mbar either within an RF ion guide or within a dedicated gas collision cell.
It is also known to use an RF ion guide to confine ions radially within an ion mobility separator or spectrometer. Ion mobility separation with RF confinement may be carried out at pressures in the range 10−1 to 10 mbar.
Different forms of RF ion guide are known including a multi-pole rod set ion guide and a ring stack or ion tunnel ion guide. A ring stack or ion tunnel ion guide comprises a stacked ring electrode set wherein opposite phases of an RF voltage are applied to adjacent electrodes. A pseudo-potential well is formed wherein the minimum of the pseudo-potential well lies along the central axis of the ion guide. Ions are confined radially within the ion guide. The ion guide has a relatively high transmission efficiency.
It is known that ion guides and ion tunnels may also be used as linear ion traps.
Ion trapping devices are widely used in mass spectrometry both as components in tandem instruments and as standalone analytical devices. There are several different types of conventional analytical traps including 3D ion traps, Paul ion traps, 2D ion traps, linear ion traps, Orbitrap® devices and FTICR devices.
Most of these devices are high resolution devices. However, there are many applications where a simple low resolution ion trap will be of great benefit. For example, if the second quadrupole (MS2) of a tandem quadrupole mass spectrometer is operated in a scanning mode then the duty-cycle of the instrument will be dramatically reduced, since the narrow resolving mass window of the second quadrupole must be scanned over the desired mass range. If mass selective ejection of ions from the collision cell is synchronised with the scanned mass window of the second quadrupole then the duty-cycle can be significantly increased.
It is desired to provide an improved ion guide.